In a known pump of this type, the rotor of the electric drive motor is fixed around the central shaft which is itself mounted on the active portion itself by means of a screw, the rotor of the electric motor being made up, depending on the type of motor, of coils, and of iron or of permanent magnets. The permanent-magnet motor is generally preferred because of its efficiency, its compactness, and the stability of its speed of rotation.
It is known that, in order to obtain better delivery-rates and compression ratios, it is necessary to increase the speed of rotation of the machine, these performance levels of the machine being proportional to its speed of rotation.
Speed of rotation is limited both by the strength of the materials used to make the rotor, and also by the dynamic behavior of the rotor, which behavior is linked inter alia to the rigidity of the rotor.
Furthermore, in order to integrate pumps into equipment, such as leak detectors or gas analyzers, it is necessary to make them as small as possible. Another important factor is degassing of the constituent parts of the pump, because the limit pressure depends in part on such degassing. It is particularly necessary to minimize such degassing at the suction end.
The rotor assembly of such a known pump is difficult to make rigid, it is bulky, costly, and difficult to degas. In particular, it is difficult to obtain good rigidity for the shaft-active portion assembly, and good resistance to centrifugal forces for the permanent magnets. It is necessary to secure the magnets firmly.